Rotary vacuum drum filters are well known for industrial and municipal applications for processing pulp, sewage, and other slurries to reduce their liquid content. In the pulp and paper industry such filters are used for brownstock washing and for filtering and washing pulp slurries. The filters generally comprise a rotary vacuum drum and vat assembly for extracting fluid from the slurry and for increasing its consistency from approximately 1% to 15% solid content. Typically, the drum is mounted for rotation in the vat with a filtering element such as a filter cloth provided about the drum. In the filtration cycle the drum emerges from the slurry at the feed end of the vat with a layer or web of filter cake on its surface and passes through a wash zone where, in the case of processing brownstock, soda is washed from the pulp. Next the drum passes through a vacuum drainage zone where a vacuum source is applied to the interior of the drum and by appropriate valving to a sector of the drum surface. In the vacuum drainage zone the evacuated drum draws liquid from the pulp or filter cake through interior piping for external recovery. Thereafter the drum rotates through a vacuum cut-off zone and a discharge zone where the pulp or filter cake is removed from the drum by a doctor blade and the cycle is repeated.
For efficient operation of the rotary vacuum drum filter it is necessary that the filter cake be substantially dewatered to a consistency of approximately 15% prior to removal and that the filter cake be of uniform consistency.
To aid in dewatering the filter cake during the filtration cycle vacuum expression devices are used which comprise an endless impervious expression belt extending across the vacuum drainage zone of the filter drum and engaging the filter cake for a portion of the circumference of the drum. A vacuum expression belt operates by means of the pressure differential created during the vacuum portion of the filtration cycle and presses the filter cake against the drum to aid in filtrate removal. The belt is mounted over spaced support rollers and positioned by suitable supporting structures in longitudinal alignment with the drum and with the inner run of the belt substantially parallel to the drum surface. The belt itself is not independently driven, however, it is carried along by engagement of the belt with the filter cake carried on the surface of the drum. The vacuum expression belt is a lamination including an interior nylon carcass with inner and outer rubber layers. The belt is preferably joined by a lap joint diagonally extending across the belt surface.
The confronting surfaces of the belt and the drum are never completely parallel because of the substantial width of the vacuum expression belt in comparison to its length in the direction of drum rotation, limitations inherent in machine design and adjustment, and variations in the thickness of filter cake during operation. As a result the vacuum expression belt tends to migrate or drift to one end or the other of the rotating vacuum filter drum. If the belt drifts off its rollers during operation there is danger of direct loss of the belt as well as consequential loss of production time while the belt is being replaced. Ordinarily, rotary vacuum drum filters operate in a harsh environment making belt replacement arduous and hazardous. It is very desirable that belt replacement occur only during scheduled periods for equipment maintenance or overhaul.
The present invention is directed to an apparatus for mounting a vacuum expression belt on a rotary vacuum filter system, for monitoring belt position during operation, adjusting the position of the apparatus as necessary to maintain the belt in normal operating position, and for inactivating the belt if unusual operating conditions occur.